Output Transformer Substitution

[bobtait]

I am going to rebuild an amp into the Robinette 6v6jcm800. His design states to use a 7K 8ohm output transformer and I already have an 8K 18 watt Mojotone tapped output transformer. I have read that the substitution is probably close enough. The plan has a fixed bias so I can set it anywhere I want. Any ideas on bias voltage?

[Bieworm]

I am going to rebuild an amp into the Robinette 6v6jcm800. His design states to use a 7K 8ohm output transformer and I already have an 8K 18 watt Mojotone tapped output transformer. I have read that the substitution is probably close enough. The plan has a fixed bias so I can set it anywhere I want. Any ideas on bias voltage?

Bias voltage needs to be set in relation to the current draw of the power tubes. It's no fixed number because of other variables.
When you go fixed bias it's a good thing to put 1 Ohm 1% resistors between pin 8 and ground on the 6V6 tubes. That makes it a lot easier to set the bias. On 6V6 amps I usually bias between 50 an 60% dissipation. Mainly because I always build tremolo into my amps and that requires colder bias settings. I also like the sound more with colder bias settings on fixed bias amps.

[bobtait]

Thank you for the reply! Did you have any thoughts on the output transformer substitution I mentioned? Also, is it better for each tube to have it's own bias pot or 1 bias pot for the pair? I am musing many things on this build. Like a switchable sag resistor on the SS rectifier. I like the thought of all the different tube possibilities but it seems I would be sacrificing different ohm cabinets and stuck always with 8 ohm speakers.

[Bieworm]

The 8k 18 W OT is frequently used for that type of amp. So that is just fine.
Bias both tubes with one bias pot. No need to have 2 bias pots unless your tubes are severely unmatched.
Why do you want a SAG resistor on an 800 circuit? The originals were/are SS too. You want that amp as tight as possible.

[JMPGuitars]

What if he wants to play blues and metal?

[Bieworm]

LOL! Build another amp for blues , I guess

[bobtait]

All of ya'lls replies are great. The reason I was considering a sag resister was if I could put it on a switch it might be cool to go back and forth between the two. I am still looking for a power transformer that has the bias voltage included. I never know what kind of music I will be playing. I spent 45 years as a bass player for all kinds of outfits but I started out on guitar so when I retired I went back. In my blues band I always use 2 amps stacked. On the bottom it's a 5e3 and on the top its a 18 watt blues breaker. Ii like the way they sound together. I have several setups that are configured like that so I can leave one at practise, one at home, etc. This 800 build I am doing is because I managed to get a mojotone hi power tweed 2-12 cabinet for like 200 bucks total with a tuki cover. and I picked up what was supposed to be an 800 clone with 6v6 but, all I can really use is the chassis and faceplates and any other components that are right. But it mounts great in that cabinet. And I figured the Robinette design would work well. I have been jonesing to recreate the last Marshall I played before I became a bass player which was a 30 watt Bluesbreaker head in a small cabinet. Just a itch, as it were. In reality the Fender and Marshall amps blended get the sound I really like. But new builds always find a way to be used. I appreciate all your input.